FIG. 10 illustrates the structure of a conventional clutch brake type steering control system. In FIG. 10, reference numeral 1 denotes an engine; 2 denotes a torque converter unit including a torque converter 2a and a lock-up clutch 2b; 3 denotes a speed change gear having forward and reverse clutches F and R, and first, second and third gear clutches 3a, 3b and 3c; 4 denotes a steering unit which includes an input shaft 5, a speed change gear mechanism 6, a horizontal input shaft 7, right and left clutches 8R and 8L, intermediate shafts 9R and 9L, brakes 10R and 10L, and output shafts 11R and 11L; 12R and 12L respectively denote right and left last reduction gears; 13R and 13L respectively denote right and left output shafts; and 14R and 14L respectively denote right and left drive sprockets. Reference numeral 18 denotes a control pump which supplies hydraulic fluid to the clutch operation valves 49L, 49R, and to the brake operation valves 50L, 50R. The steering of the bulldozer is performed by the components located between the engine 1 and the drive sprockets 14R and 14L.
FIG. 11 schematically shows a control device for both the right and left clutches 8R and 8L and the right and left brakes 10R and 10L. Reference numeral 15R denotes a manually operated right track steering lever; 15L denotes a manually operated left track steering lever; 16 denotes a brake pedal; 46R and 46L denote steering lever arms; 47R and 47L denote brake lever arms; 49R and 49L denote clutch operation valves for supplying a controlling hydraulic pressure to the right and left clutches 8R and 8L in accordance with the amount of operation of the right track and left track steering levers 15R and 15L, respectively; 50R and 50L denote brake operation valves for supplying a controlling hydraulic pressure to the right and left brakes 10R and 10L, respectively; 51R and 51L denote steering links; 52 denotes a rod; 53 denotes a brake link; 54 denotes a returning spring for the rod 52; and 55R and 55L denote adjusting bolts.
The control method of the structure shown in FIGS. 10 and 11 is described below. As the right track steering lever 15R is operated in a direction indicated by the reference character A during the running of the bulldozer, it starts pushing a rod 49r of the clutch operation valve 49R via the steering link 51R and the steering lever arm 46R. As the clutch operation valve 49R starts supplying the control pressure to the right clutch 8R, the right clutch 8R starts becoming disengaged. As the right steering lever 15R is further operated in the direction indicated by the reference character A, the right clutch 8R becomes completely disengaged and the steering lever arm 46R starts pushing a rod 50r on the brake operation valve 50R. As the control pressure of the right brake 10R starts decreasing, the right brake 10R starts being engaged.
When the brake pedal 16 is depressed during the rightward steering operation, the rod 52 is pushed down against the spring 54, pushing the rods 50r and 50l of the right and left brake valves 50R and 50L via the brake link 53 and then the right and left brake lever arms 47R and 47L, whereby the right and left brakes 10R and 10L can be engaged in accordance with the amount of operation of the brake pedal 16. At that time, the right brake operation valve 50R is controlled in accordance with whichever is the largest: the amount of operation of the rod 50r of the right brake operation valve 50R by the right track steering lever 15R, or the amount of operation of the rod 50r of the right brake operation valve 50R by the brake pedal 16.
As the movement of the left track steering lever 15L in a direction indicated by the reference character A results in an operation similar to that performed when the right track steering lever 15R is moved in the direction indicated by the reference character A, the description thereof is omitted.
FIGS. 12 and 13 show the modulation characteristics between a clutch and the associated brake when the clutch is turned off (disengaged) and the brake is turned on (engaged). The ordinate axis represents the hydraulic pressure of a hydraulic oil, while the abscissa axis represents the time during the operation of the respective steering lever.
FIG. 12 shows how the hydraulic pressure for the right clutch 8R and the hydraulic pressure for the right brake 10R change with time when the right track steering lever 15R is operated slowly. As can be seen from FIG. 12, the right brake 10R starts being engaged after the right clutch 8R has been disengaged. Thus, the hydraulic pressure for the clutch and that for the brake change smoothly. This shows that no shock is given to the vehicle.
FIG. 13 shows changes in the hydraulic pressure when the right track steering lever 15R is operated rapidly. As can be seen from FIG. 13, the right brake 10R starts being engaged before the right clutch 8R has been completely disengaged. Thus, the hydraulic pressure for the brake changes greatly. This shows that a large shock is given to the vehicle. To prevent generation of the shock, the steering lever arms 46R and 46L are respectively provided with the adjusting bolts 55R and 55L. The adjusting bolts 55R and 55L are used to adjust the operation interval between the clutches 8R, 8L and the brakes 10R, 10L, and hence provide a time interval between the release of the clutch and the engagement of the brake or between the release of the brake and the engagement of the clutch.
However, when the steering operation is performed at an inclined site, the clutch and the brake, which are provided for the crawler track on the side of the vehicle to which the steering lever is moved, are disengaged at the same time for a certain period of time. As a result, the crawler track provided on the side of the vehicle to which the steering lever is operated travels by its weight, and the vehicle thus turns in a direction opposite to that in which the steering lever is moved, i.e., a so-called reverse steering phenomenon occurs. This may be prevented by engaging the brake before the clutch is disengaged or by engaging the clutch before the brake is released. However, this results in the clutch and the brake being engaged at the same time, generating a shock when the steering lever is operated during running and thus deteriorating the comfortable riding of the operator.